The present invention relates to a magnetic core for use in a saturable reactor, a multi-output switching regulator controlling the output voltage by a magnetic amplifier, and a computer equipped with such a multi-output switching regulator.
The multi-output switching regulator has been used in personal computers and office computers. For example, in a PC AT-X type computer, a most typical desktop personal computer, a multi-output switching regulator with five outputs, i.e., +5V output (1.5-20 A), +3.3V output (0-20 A), +12V output (0.2-8 A), -5V output (0-0.3 A) and -12V output (0-0.4 A) is used when a larger output capacity is required. In the above five-output switching regulator, the main circuit comprises a forward converter with single switching element or a half bridge converter. The main output (+5V output) is controlled by a pulse-width modulation of a switching element located in a primary side of a main transformer, and the secondary outputs (+3.3V, +12V, -5V and -12V outputs) are controlled at the secondary side of the main transformer.
One of the methods for controlling the secondary outputs at the secondary side of the main transformer is a control by a magnetic amplifier located at the secondary side of the main transformer. The magnetic amplifier basically comprises, as the main components, a saturable reactor, a diode and an error amplifier. This method has advantages of simultaneously attaining a small size, a high efficiency, a low noise generation and a high reliability, which have not been attained by a control using a chopper circuit and a dropper circuit utilizing semiconductor elements. It has been known in the art that the control by the magnetic amplifier is advantageous for controlling the output with a low voltage and a large load current, particularly in view of a high efficiency, because the loss in the saturable reactor serve as a control element is small as compared with the loss in the semiconductor control element used in the chopper circuit or the dropper circuit even when the load current is large. Therefore, in the multi-output switching regulator for the PC AT-X type personal computer, the magnetic amplifier has been widely used for controlling the +3.3 V and +12 V outputs having a large load current. In the present invention, the switching regulator utilizing the magnetic amplifier is referred to as a magnetic amplifier type switching regulator.
The switching frequency of the magnetic amplifier type multi-output switching regulator is usually set to about 50-200 kHz. Therefore, a Co-based amorphous core has been widely used as the magnetic core for the saturable reactor of the magnetic amplifier. However, in the magnetic amplifier type multi-output switching regulator incorporated with a saturable reactor having the Co-based amorphous core, the secondary output voltage being controlled by the magnetic amplifier is lower than the reference value due to the voltage drop by the saturable reactor when the load current increases even if the reset current Ir for the saturable reactor is made zero. The output voltage drop is attributable to a residual operating magnetic flux density .DELTA.Bb of the core and an unfavorable reset of the saturable reactor by a reverse recovery current Irr from a diode connected in series to the saturable reactor.
The voltage drop by the saturable reactor increases with increasing residual operating magnetic flux density .DELTA.Bb when the core size and the number of turns of the saturable reactor are constant. Also, the magnetic flux density .DELTA.Br to be reset by the reverse recovery current Irr from the diode is larger in a core which acquires a larger control magnetic flux density .DELTA.B by a small control magnetizing force when the core size and the number of turns of the saturable reactor are constant.
In this connection, it has been known in the art that the voltage drop by the saturable reactor is smaller in using an anisotropic 50%-Ni permalloy core than in using the Co-based amorphous core when the core size and the number of turns of the saturable reactor are the same, because the anisotropic 50%-Ni permalloy core shows a small residual operating magnetic flux density .DELTA.Bb and acquires a smaller control magnetic flux density .DELTA.B when magnetized by the same control magnetic force as applied to the Co-based amorphous core. However, since the anisotropic 50%-Ni permalloy core shows a large core loss at a higher frequency range, the switching frequency is limited to about 20 kHz at most, and it has been recognized in the art that the use of the anisotropic 50%-Ni permalloy core at a switching frequency higher than 20 kHz has been impractical, because such a use requires an extremely increased number of turns and causes a significant temperature rise of the saturable reactor. Therefore, the anisotropic 50%-Ni permalloy core fails to reduce the size of the magnetic amplifier type multi-output switching regulator and is not suitable for the application such as a personal computer which requires a reduced size.
In the present invention, .DELTA.B, .DELTA.Bb and .DELTA.Br are defined as shown in FIG. 5, wherein Br is a residual magnetic flux density, H is a control magnetizing force, and H.sub.Lm is the maximum value of a gate magnetizing force.
In the magnetic amplifier type multi-output switching regulator, for example, used in the PC AT-X type desktop personal computer, both the main output (+5V output) and the secondary output (+3.3V output) are usually taken out of the same secondary winding of the transformer, because the potential difference between the +5V output and the +3.3V output is small. Therefore, it has been known that the voltage drop in the +3.3V output cannot be avoided by using a secondary winding for the +5V output and another secondary winding for the +3.3V output with a number of turns larger than that of the secondary winding for the +5V output.
To eliminate the above disadvantage, Japanese Patent Publication No. 2-61177 discloses a magnetic amplifier in which a reset circuit comprising series-connected rectifying diode and control element is connected in parallel to both the ends of a saturable reactor, thereby to control the reset of the saturable reactor by the control element. However, the proposed magnetic amplifier requires at least four additional circuit elements to spoil the advantage such as a small number of circuit elements of the magnetic amplifier type multi-output switching regulator.
Japanese Patent Laid-Open No. 63-56168 discloses a magnetic control type switching regulator in which a saturable reactor has a winding for forming a short circuit in addition to a main winding for output, thereby to avoid the drop in the output voltage attributable to a dead time and an unfavorable reset of the saturable reactor by the reverse recovery current Irr of a rectifying diode. However, the proposed method is insufficient in preventing the voltage drop of the saturable reactor as compared with the method disclosed in Japanese Patent Publication No. 2-61177, because the additional winding for the short circuit, an additional diode serving as an active element in the short circuit and the reverse recovery current from the additional diode cause the voltage drop of the saturable reactor.
Japanese Patent Publication No. 7-77167 discloses a magnetic core made of an Fe-based alloy containing Fe, Cu and M as essential components, wherein M is at least one element selected from the group consisting of Nb, W, Ta, Zr, Hf, Ti and Mo. It is described that the saturable reactor made of the proposed magnetic core has a high squareness ratio and shows a small core loss and a high magnetic flux density. However, the proposed magnetic core shows an increased .DELTA.Bb due to the impact or shock thereon during the production process, and this problem has not been avoided by the production method disclosed therein. Therefore, a magnetic amplifier type multi-output switching regulator utilizing a saturable reactor made of the proposed magnetic core generates an output voltage lower than the reference value when the load current is large.